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The invention relates to a method for setting an electric motor with respect to a reference position corresponding to a mechanical stop transmitted, through a reduction gear, to said electric multi-phase motor with sequential power supply for step-by-step operation.
The peculiarity of a stepper motor is that it ensures a continuous incremental mechanical motion. That is why they are often used in measuring systems for carrying out indicating functions. As a typical application, one should note the control of pointers of measuring or indicating instruments, such as the instruments of a dashboard of a motor vehicle or the control of shutters for adjusting the air flow in an air-conditioning circuit.
In a stepper motor, to each power-supply impulse corresponds a constant elemental rotation by one step. A determined number of impulses results into a corresponding number of steps and, hence, into a known rotation of the rotor. The succession of impulses at a determined frequency allows imparting an almost constant speed of rotation.
In fact the stepper motor is often associated with a gear system that acts as a reduction gear in order to mechanically reduce the amplitude of the angular pitch at the level of the load, for instance a pointer. In that way, the resolution is increased.
In an indicating system with a sufficiently good resolution, one cannot see the incremental nature of the motor""s rotation, which gives the impression of a continuous motion at the level of the pointer.
FR-95.15436 and FR-96.12765 or also EP-A-587,685 already now disclose such multi-phase rotary stepper motors, namely for controlling a pointer of an indicating device.
In this respect one understands that it is important in this field of application to have a good matching between the pointer""s position and the graduation of the dial in front of which the latter has to move. One of the difficulties experienced at the putting into operation of an apparatus of this kind is the location of the pointer with respect to a reference position, in particular when there is no indication about the initial position of the pointer or also when an offset has occurred due to a failure during the pause phase that has resulted in a loss of information. The system must then be re-initialized, in order to cause the pointer to start again from a known position.
It is therefore necessary to fix a reference position likely to define the angular origin of the rotor, so that by counting the number of steps performed, starting from the reference position, thus the number of impulses applied to the motor, it is possible to know the angular position of the rotor with respect to this reference. Of course, knowing the characteristics of the reduction gear, it is possible to derive from it the position of the controlled pointer, even of a shutter, except for the backlash of the gear.
Since in a large number of applications regarding an indicating device with a pointer, the latter must rotate according to a travel path of less than 3601, even of less than 3001, the reference position can be defined by a mechanical stop located either at the level of the dial in front of which said pointer moves or also at the level of the gear system, so as to limit the angular travel distance of the tooth-wheels the latter is comprised of. In the case of a reduction gear, the internal mechanical stop is usually located on the end wheel, i.e. the one the axis of which controls the load, hence, the pointer.
Therefore, to proceed to this initialization of the system, one controls the power supply to the motor, so that its rotor controls the rotation of the load towards the mechanical stop. Once the latter has been detected and knowing the angle between this mechanical stop and the zero value of the dial, one applies the number of steps required in order to bring the pointer to this zero value. Likewise, during the operation of the indicating system, it is possible to transmit to the motor the power-supply sequences or impulses resulting into a determined number of steps, in order to impart to the pointer the necessary angular position along the indicating dial.
Once more, it should be clearly stated that, though the state of technique described in particular for setting forth the problem that arises is that related to the indicating devices with a pointer, the present invention is in no way limited, since it will also find an application for other devices such as the devices for controlling the shutters of air-conditioning circuits for motor vehicles. Thus, the invention will be of interest whenever it is necessary, despite an operation in open circuit and, hence, in the absence of sensors, to know the accurate position of a load driven by a stepper motor, in particular through a reduction gear.
Finally, this re-initialization of the indicating system has to be performed at each putting into operation, which means that the reference position is searched for at each switching-on.
In this respect EP-A-0,551,562 discloses a method for setting a multi-phase electric stepper motor with respect to a mechanical stop. This method is based on the principle that from the action of the rotation of the rotor including N pairs of poles results an induced voltage in the phases corresponding to the coiling of the stator. When the rotor stops rotating, this voltage is obviously canceled.
Therefore, this previously known method merely consists in detecting the zero crossing of the voltage induced into at least one of the phases of the motor, in order to know the reference position.
This kind of method has a number of drawbacks. In particular, in the case of an indicating system, the kick back of the pointer cannot be avoided and will lead to an error in the position of the latter and, hence, to a poor indicating accuracy.
In addition, for reasons of reliability of the method, it is common practice to feed the motor in a reverse direction of rotation with respect to the mechanical stop, before causing it to reverse the direction to finally detect this mechanical stop through measuring an induced voltage equal to zero. In the case of an indicating device, this movement of the pointer without any apparent reason to the user has to be avoided.
Since this induced voltage is measured at the level of one phase of the motor during a sequence in which said phase is not fed, it is absolutely necessary to monitor d-ds zero crossing of the induced voltage on each phase of the motor if one wants to accurately detect, to within one step, the reference position corresponding to said mechanical stop.
Obviously, such a reference position could be detected by any other means, namely through an optical sensor capable of detecting the crossing of the pointer of a shutter or the like. Such solutions are however cumbersome and expensive.
The present invention is based on the fact established during a first inventive step, that the presence of the gear system, such as a reduction gear, beyond which is necessarily located the mechanical stop, is the reason for there existing a determined period of time between the moment at which the device meets this mechanical stop and that at which the motor stops, this transient state namely resulting from the mechanical backlash and the elasticity this kind of mounting exhibits. More exactly, when the end wheel of the reduction gear or the driven pointer arrives at the stop, the various wheels and tooth-wheels of this gear system are progressively blocked against each other with a determined stress, which results into a progressive increase of the load of the motor, in particular, a slowing down of the latter, which results into a change in the induced voltage in the phases of this motor from the value of this induced voltage during stabilized operation to zero. Therefore, the object of the present invention consists in a method the principle of which is, as a matter of fact based on the detection of this change of this induced voltage and/or of its crossing below a threshold value, this under well determined conditions, to instantaneously control the stoppage of the motor, i.e. even before this induced voltage is equal to zero.
Finally, by stopping the sequence of power supply to the motor as soon as possible, well before the induced voltage crosses the zero point, the gear is impeded from fully blocking, which would results into the kick back of the pointer or of the shutter on the mechanical stop and, hence, into a positioning error. One should note that since this transient phenomenon resulting from the transmission of the effect of the mechanical stop to the motor through the gear system occurs irrespective of whether the load is far from the reference position or is close to it, it is in any case possible to detect this transient state of the induced voltage in a phase of the motor and, through it the reference position searched for.
To this end, the invention relates to a method for setting an electric motor with respect to a reference position corresponding to a mechanical stop transmitted, through a gear system, namely a reduction gear, to said electric multi-phase motor with sequential power supply for step-by-step operation, characterized in that it consists in measuring in a determined time window, during a sequence of non-feeding of at least one of the phases of the motor, the induced voltage in the phase that is not fed and in comparing its average value with a threshold value, in order to stop the feeding of said motor on the sequence in progress if this average measured value is lower than said threshold value. One should note that the chosen threshold value should be chosen lower than the average value that is normally reached by this induced voltage at the speed that is imposed to the controlled load.
The invention also relates to a method for setting an electric motor with respect to a reference position corresponding to a mechanical stop transmitted, through a gear system, namely a reduction gear, to said electric multi-phase motor with sequential power supply for step-by-step operation, characterized in that it consists in reading in a determined time window, during a sequence of non-feeding of at least one of the phases of the motor, the change in the induced voltage in the phase that is not fed and in comparing that change with a standard change, in order to stop the feeding of said motor on the sequence in progress in the event of an offset.